The Character of South Pole-Aitken Basin: Patterns of Surface and Subsurface Composition

Using Moon Mineralogy Mapper data, we characterize surface diversity across the enormous South Pole-Aitken Basin (SPA) by evaluating the abundance and composition of pyroxenes, which are overwhelmingly the most abundant mafic mineral in the region. Although SPA exhibits significant complexity due to billions of years of geologic processes subsequent to formation, the basin has retained regular patterns of compositional heterogeneity across its structure. Four distinct, roughly concentric zones are defined: (1) a central SPA compositional anomaly, which exhibits a pervasive elevated Ca,Fe-rich pyroxene abundance; (2) a Mg-pyroxene annulus, which is dominated by abundant Mg-rich pyroxenes; (3) a heterogeneous annulus, which exhibits localized pyroxene-rich areas spatially mixed with feldspathic materials; and (4) the SPA exterior, which is primarily feldspathic. Pyroxene compositions in the heterogeneous annulus are similar to those in the Mg-pyroxene annulus, and Mg-rich pyroxenes also underlie the more Ca,Fe-rich pyroxene surface material across the SPA compositional anomaly. The establishment of these four distinct compositional zones across SPA constrains basin evolution models and serves to guide potential sample return (and other science) targets. Plain Language Summary The South Pole-Aitken Basin is a vast impact structure on the far side of the Moon. In this study, we look at the compositional structure of the basin using data from orbital instruments. We find that this massive basin exhibits several distinct compositional zones. These zones arise from incredibly energetic processes associated with the impact event, which excavated and melted rocks from deep beneath the lunar surface. Later in lunar history, volcanic processes and additional impacts further shaped the basin interior. We identify an anomalous similar to 650km region in central South Pole-Aitken, which exhibits an unusual composition, raising questions about its origin. These results have implications for our understanding of lunar evolution and the formation of large impact basins. Additionally, these results may help in the selection of landing sites for anticipated sample return and other science missions.